docs: def API520_W(Pset, Pback):
      r'''Calculates capacity correction due to backpressure on balanced
      spring-loaded PRVs in liquid service. For pilot operated valves,
      this is always 1. Applicable up to 50% of the percent gauge backpressure,
      For use in API 520 relief valve sizing. 1D interpolation among a table with
      53 backpressures is performed.
  
      Parameters
      ----------
      Pset : float
          Set pressure for relief [Pa]
      Pback : float
          Backpressure, [Pa]
  
      Returns
      -------
      KW : float
          Correction due to liquid backpressure [-]
  
      Notes
      -----
      If the calculated gauge backpressure is less than 15%, a value of 1 is
      returned.
  
      Examples
      --------
      Custom example from figure 31:
  
      >>> API520_W(1E6, 3E5) # 22% overpressure
      0.9511471848008564
  
      References
      ----------
      .. [1] API Standard 520, Part 1 - Sizing and Selection.
      '''
      gauge_backpressure = (Pback-atm)/(Pset-atm)*100.0 # in percent
      if gauge_backpressure < 15.0:
          return 1.0
      return interp(gauge_backpressure, Kw_x, Kw_y)